Portable computers, such as laptop or notebook computers, are powered either by an alternating current (AC) power source or a battery power source. In general, it is desirable to reduce the current required to operate a computing or processor system. Reduction in current consumption by portable computers is particularly desirable because reduced current consumption results in longer battery life.
It has been established that the power consumption of a processor is proportional to the product of the processor's dynamic capacitance (C), the clock rate at which the processor is operating (f), and the square of the processor's operating or supply voltage (v2). However, a processor operating at fast clock speeds requires a higher operating voltage than that same processor operating at a slower clock speed. Thus, to minimize the power consumption of a processor while a computer is operating under battery power, it possible to reduce the clock speed (f) and the operating voltage (v) of the processor, while maintaining acceptable operating performance of the processor. This sacrifices processing speed for extended battery life. Of course, it is not desirable to reduce the operating frequency or operating voltage to a point at which processor ceases to function properly (i.e., to cause the processor to operate in an out-of-state mode of operation). Thus, it is possible to determine voltage/frequency pairs, or two-tuples of operating parameters, at which processor operation is acceptable. As will be readily appreciated by those having ordinary skill in the art, the operating parameters defining acceptable operation for a first processor do not necessarily define acceptable operation for a second processor because each processor is slightly different in its processing parameters, which results in slightly different operating parameters and, thus, different voltage/frequency pairs at which processor current consumption is low but processor operation is acceptable.
Based on the differences between the operating parameters of various processors and the desire to determine low current consumption, voltage/frequency pairs of operating parameters for each processor are determined at the factory before the processor ships. For example, a processor is tested at various voltage/frequency pairs to determine acceptable operating parameters (i.e., frequency/voltage pairs or two-tuples at which processor operation is acceptable). The operating parameters (i.e., voltage/frequency two-tuples) yielding acceptable processor operation are then stored in model specific registers (MSR) on that processor.
During subsequent operation of the processor, a basic input/output system (BIOS) operating on the processor retrieves the operating parameters stored in the MSR and calculates a straight-line approximation to determine a straight voltage/frequency line along which the processor can operate so as to reduce current consumption when slower processor operating speeds are acceptable. Once the straight line is calculated based on the operating parameters, the BIOS determines a set of tables referred to as power state (P-state) tables in the advanced configuration and power interface (ACPI). The P-state ACPI tables may be used by an operating system power management (OSPM) entity, such as acpi.sys or other Microsoft Windows® drivers, to determine power reduced processor operating states that the processor can reliably support.
However, as will be readily appreciated by those having ordinary skill in the art, not all processors have linear voltage/frequency operating characteristics. In fact, most often acceptable operating voltage/frequency pairs have a square-law relationship. Any deviation from the calculated voltage/frequency line is error, which can take two forms: either disadvantaging the systems because it will not go to its lowest power state or, leading in the OSPM to direct the system to an out-of-state mode of operation.